This invention relates to a centrifugal separator for a three-phase mixture of sludge, heavy liquid and light liquid, and more specifically to an automatic sludge discharging centrifugal separator including means for detecting the level of sludge accumulated in a rotary structure, and means responsive thereto for causing the structure to periodically discharge the sludge.
In conventional automatic sludge discharging centrifugal separators, high concentrations of sludge are desirably accumulated exactly up to an intended volume level in a rotary structure, and then the sludge is discharged. It is difficult, however, to accurately detect the volume of sludge deposited in the rotary structure. Typically, the sludge discharge is therefore carried out at certain time intervals on a trial and error basis, or after the quantity of sludge deposited has already become excessive. A variety of methods have been proposed in the prior art for detecting the sludge accumulation level. For example, one method utilizes a phenomenon in which a liquid passage is closed by the surface of sludge deposited in the rotary structure approaching the rotation center. See, for instance, Japanese Patent Publication Nos. 24490/1963, 29839/1969 and 25351/1973. Another method utilizes vibration generated in the rotary structure by the increase of the sludge level. See, for instance, Japanese Pat. Publication No. 28837/1969. Still another method detects the sludge level in accordance with the up and down movement of a float due to the specific gravity difference, as in Japanese Pat. Publication No. 43345/1971. A final method detects the sludge level by utilizing the circulation of liquid due to the specific gravity difference, as in Japanese Pat. Publication No. 3401/1973. However, these methods suffer from the following difficulties: (1) since the sludge level detecting means is typically connected through an electrical mechanism to the sludge discharging means, if the centrifugal separator is operated under severe conditions, the electrical mechanism is liable to be damaged, and besides this, the installation cost is expensive; and (2) the conventional methods mentioned above are applicable to a two-phase separation in which sludge is removed from liquid, but are not effective in a three-phase separation in which sludge, heavy liquid and light liquid are recovered. In a method in which such an electrical mechanism is eliminated (See, for example, Japanese Pat. Publication No. 19833/1973), liquid introduced into a pressure chamber to discharge the sludge itself contains some sludge, and therefore a nozzle used for removing the liquid from the pressure chamber is liable to become clogged.